In general, a compressor that is a mechanical apparatus for increasing a pressure, by receiving power from a power unit system such as an electric motor or turbine and compressing air, refrigerants or other various operation gases has been widely used for home appliances such as a refrigerator and an air conditioner or in the whole industrial fields.
The compressors are roughly divided into a reciprocating compressor having a compression space through which operation gases are sucked or discharged between a piston and a cylinder, so that the piston can be linearly reciprocated inside the cylinder to compress refrigerants, a rotary compressor having a compression space through which operation gases are sucked or discharged between an eccentrically-rotated roller and a cylinder, so that the roller can be eccentrically rotated on the inner walls of the cylinder to compress refrigerants, and a scroll compressor having a compression space through which operation gases are sucked or discharged between an orbiting scroll and a fixed scroll, so that the orbiting scroll can be rotated with the fixed scroll to compress refrigerants.
Recently, among the reciprocating compressors, a linear compressor has been mass-produced because it has high compression efficiency and simple structure by removing mechanical loss by motion conversion by directly connecting a piston to a driving motor performing linear reciprocation.
Generally, the linear compressor which sucks, compresses and discharges refrigerants by using a linear driving force of the motor includes a compression unit consisting of a cylinder and a piston for compressing refrigerant gases, and a driving unit consisting of a linear motor for supplying a driving force to the compression unit.
In detail, in the linear compressor, the cylinder is fixedly installed in a closed vessel, and the piston is installed in the cylinder to perform linear reciprocation. When the piston is linearly reciprocated inside the cylinder, refrigerants are sucked into a compression space in the cylinder, compressed and discharged. A suction valve assembly and a discharge valve assembly are installed in the compression space, for controlling suction and discharge of the refrigerants according to the inside pressure of the compression space.
In addition, the linear motor for generating a linear motion force to the piston is installed to be connected to the piston. An inner stator and an outer stator formed by stacking a plurality of laminations at the periphery of the cylinder in the circumferential direction are installed on the linear motor with a predetermined gap. A coil is coiled inside the inner stator or the outer stator, and a permanent magnet is installed at the gap between the inner stator and the outer stator to be connected to the piston.
Here, the permanent magnet is installed to be movable in the motion direction of the piston, and linearly reciprocated in the motion direction of the piston by an electromagnetic force generated when a current flows through the coil. Normally, the linear motor is operated at a constant operation frequency fc, and the piston is linearly reciprocated by a predetermined stroke S.
On the other hand, various springs are installed to elastically support the piston in the motion direction even though the piston is linearly reciprocated by the linear motor. In detail, a coil spring which is a kind of mechanical spring is installed to be elastically supported by the closed vessel and the cylinder in the motion direction of the piston. Also, the refrigerants sucked into the compression space serve as a gas spring.
The coil spring has a constant mechanical spring constant Km, and the gas spring has a gas spring constant Kg varied by load. A natural frequency fn of the piston (or linear compressor) is calculated in consideration of the mechanical spring constant Km and the gas spring constant Kg.
The thusly-calculated natural frequency fn of the piston determines the operation frequency fc of the linear motor. The linear motor improves efficiency by equalizing its operation frequency fc to the natural frequency fn of the piston, namely, operating in the resonance state.
Accordingly, in the linear compressor, when a current is applied to the linear motor, the current flows through the coil to generate an electromagnetic force by interactions with the outer stator and the inner stator, and the permanent magnet and the piston connected to the permanent magnet are linearly reciprocated by the electromagnetic force.
Here, the linear motor is operated at the constant operation frequency fc. The operation frequency fc of the linear motor is equalized to the natural frequency fn of the piston, so that the linear motor can be operated in the resonance state to maximize efficiency.
As described above, when the piston is linearly reciprocated inside the cylinder, the inside pressure of the compression space is changed. The refrigerants are sucked into the compression space, compressed and discharged according to changes of the inside pressure of the compression space.
The linear compressor is formed to be operated at the operation frequency fc identical to the natural frequency fn of the piston calculated by the mechanical spring constant Km of the coil spring and the gas spring constant Kg of the gas spring under the load considered in the linear motor at the time of design. Therefore, the linear motor is operated in the resonance state merely under the load considered on design, to improve efficiency.
However, since the actual load of the linear compressor is varied, the gas spring constant Kg of the gas spring and the natural frequency fn of the piston calculated by the gas spring constant Kg are changed.
In detail, as illustrated in FIG. 1A, the operation frequency fc of the linear motor is determined to be identical to the natural frequency fn of the piston in a middle load area at the time of design. Even if the load is varied, the linear motor is operated at the constant operation frequency fc. But, as the load increases, the natural frequency fn of the piston increases.
                              f          n                =                              1                          2              ⁢                                                          ⁢              π                                ⁢                                                                      K                  m                                +                                  K                  g                                            M                                                          Formula        ⁢                                  ⁢        1            
Here, fn represents the natural frequency of the piston, Km and Kg represent the mechanical spring constant and the gas spring constant, respectively, and M represents a mass of the piston.
Generally, since the gas spring constant Kg has a small ratio in the total spring constant Kt, the gas spring constant Kg is ignored or set to be a constant value. The mass M of the piston and the mechanical spring constant Km are also set to be constant values. Therefore, the natural frequency fn of the piston is calculated as a constant value by the above Formula 1.
However, the more the actual load increases, the more the pressure and temperature of the refrigerants in the restricted space increase. Accordingly, an elastic force of the gas spring itself increases, to increase the gas spring constant Kg. Also, the natural frequency fn of the piston calculated in proportion to the gas spring constant Kg increases.
Referring to FIGS. 1A and 1B, the operation frequency fc of the linear motor and the natural frequency fn of the piston are identical in the middle load area, so that the piston can be operated to reach a top dead center (TDC), thereby stably performing compression. In addition, the linear motor is operated in the resonance state, to maximize efficiency of the linear compressor.
However, the natural frequency fn of the piston gets smaller than the operation frequency fc of the linear motor in a low load area, and thus the piston is transferred over the TDC, to apply an excessive compression force. Moreover, the piston and the cylinder are abraded by friction. Since the linear motor is not operated in the resonance state, efficiency of the linear compressor is reduced.
In addition, the natural frequency fn of the piston becomes larger than the operation frequency fc of the linear motor in a high load area, and thus the piston does not reach the TDC, to reduce the compression force. The linear motor is not operated in the resonance state, thereby decreasing efficiency of the linear compressor.
As a result, in the conventional linear compressor, when the load is varied, the natural frequency fn of the piston is varied, but the operation frequency fc of the linear motor is constant. Therefore, the linear motor is not operated in the resonance state, which results in low efficiency. Furthermore, the linear compressor cannot actively handle and rapidly overcome the load.
On the other hand, in order to rapidly overcome the load, as shown in FIGS. 2A and 2B, the conventional linear compressor allows the piston 6 to be operated inside the cylinder 4 in a high or low refrigeration mode by adjusting an amount of voltage (or current) applied to the linear motor. The stroke S of the piston 6 is varied according to the operation modes, to change a compression capacity.
As illustrated in FIG. 2A, a voltage V1 is used for the high refrigeration mode and a voltage V2 is used for the low refrigeration mode. When the voltages V1 and V2 have positive values from a null point (0), the piston 6 performs compression, and when the voltages V1 and V2 have negative values, the piston 6 performs suction. Here, peak values of the voltages V1 and V2 must be smaller than the maximum voltage threshold value Vp outputted from the linear compressor.
Since peak-peak values of the voltages V1 and V2 decide the stroke S of the piston 6, the stroke S of the piston 6 is controlled by changing the peak-peak values. In the high refrigeration mode, the peak-peak value of the voltage V1 is equal to the peak-peak value 2Vp according to the maximum voltage threshold value Vp, and thus the piston 6 reaches the TDC (high refrigeration mode stroke S1). In the low refrigeration mode, the peak-peak value of the voltage V2 is reduced, and thus the piston 6 is linearly reciprocated not to reach the TDC.
The linear compressor is operated in the high refrigeration mode in a state where the load is relatively large. In the high refrigeration mode, the operation frequency fc of the linear motor is equalized to the natural frequency fn of the piston 6, so that the piston 6 can be operated to reach the TDC with a predetermined stroke S1.
In addition, the linear compressor is operated in the low refrigeration mode in a state where the load is relatively small. In the low refrigeration mode, the compression capacity can be reduced by decreasing the voltage applied to the linear motor. However, in a state where the piston 6 is elastically supported in the motion direction by the elastic force of the mechanical spring and the gas spring, a stroke S2 of the piston 6 is reduced. Accordingly, the piston 6 cannot reach the TDC. Moreover, the operation frequency fc is different from the varied natural frequency fn of the piston 6, which results in low efficiency and compression force of the linear compressor.